Incorrect trap code for iOS fails compilation

**Describe the bug**
v2.11.1 [introduced support for breaking into the debugger on iOS](https://github.com/catchorg/Catch2/pull/1817) and while it works with XCode 10 and 11, it fails with 8 and 9.

You can reproduce the issue by cloning sjson-cpp at [this branch](https://github.com/nfrechette/sjson-cpp/tree/bug/catch2-ios). Make sure to clone submodules as Catch2 is used as a submodule.

`python make.py -clean -build -compiler ios -config Debug`

The issue also happens with `Release` config. See [here](https://travis-ci.org/nfrechette/sjson-cpp/builds/652285132) for example build output.

**Expected behavior**
I know that these are older xcode versions but I am wondering if we could support them as 2.11.0 worked fine previously.

**Reproduction steps**
Simply compiling Catch2 v2.11.1 on XCode 8 or 9 will fail.

**Platform information:**
- OS: **OS X 10.13.6**
- Compiler+version: **Apple LLVM version 9.1.0 (clang-902.0.39.2)**
- Catch version: **v2.11.1**

**Additional context**